During the process of manufacturing semiconductor devices, wafer temperature is an important parameter in controlling the physical properties of the material surface structure or film deposition or etching. In fact, control of the wafer temperature and uniformity of the wafer temperature are key parameters for achieving process control and uniformity. Presently, manufacturers of semiconductor devices typically use indirect methods to determine whether the temperature in the chambers is uniform. Direct temperature measurement with an instrumented substrate allows direct temperature control and optimization of temperature uniformity. Therefore, the yield of semiconductor devices can be substantially increased with an apparatus that provides the actual temperature of the regions within the substrate.
Common current practice is to use an indirect temperature measurement method that relies on measuring a temperature related change to the film properties or electrical properties of a test substrate subjected to a process thermal cycle. Since the change in test wafer properties is measured after the process is completed, only a single measurement is available for determining the process thermal history, the peak temperature reached or an indication of a time temperature integral. Moreover, indirect measurement methods require repeated process cycles using a large number of test substrates to characterize or optimize the process cycle. Consequently, since the process thermal cycle may be complex with many different temperature steps, an in-situ continuous measurement of temperature during the complete process cycle is needed.
However, use of direct in-situ temperature measurement systems, which typically employ a pattern of discrete resistive thermal detectors (RTD) or thermocouples (TC) bonded to the top of a test wafer, is limited. The RTD or TC leads of such systems are routed out of the processing chamber through either an electrical connector in a vacuum flange feedthrough, or a flat cable that can be placed under an O-ring seal. Unfortunately, external RTD or TC leads can drain heat from or conduct heat to the measurement junction or the substrate. In addition, the thermal conduction, energy absorption and emissivity properties of the bonding material used to attach the discrete sensors may create a source of error. In fact, the presence of sensor leads can attenuate the energy flowing from the heating source to the wafer, thereby altering the temperature of the wafer. Thus, the presence of sensor leads above a wafer surface can change the temperature of the wafer and provide a distorted temperature reading.
Another problem associated with known in-situ design systems is damage to the conductor wires in a flat cable caused by clamping under processing chamber O-ring flanges. Also, flat cable feedthroughs placed between a compliant O-ring and a flat sealing surface produce a low leak rate through the pressurized or vacuum seal. However, the surface of the film has less compliance than the O-ring and does not completely fill in and block gas leakage along the surface scratches and imperfections of the flange. Consequently, although the leak rate is very low on the side of the flat cable in contact with the O-ring, the leak rate is higher between the flat flange sealing surface and the film surface of the flat cable feedthrough.
Therefore, what is needed is a technique for instrumenting a substrate that maintains uniform surface emissivity and provides precise temperature measurement. In addition, a system is needed for protecting the wire conductors during repeated insertion into the vacuum chamber and from heat in excess of 400.degree. C. The present invention overcomes the problems associated with known systems by disclosing a method and apparatus for sensing temperature on a substrate in an integrated circuit fabrication tool. This results in a system that provides a uniform sensor and interconnect pattern on the wafer with optimized emissivity uniformity. As such, the system is able to determine the actual temperatures in each desired region of the wafer similar to a wafer that is not instrumented at all. Moreover, the flat cable construction allows the cable to be repeatedly placed under a vacuum seal without damaging the internal conductor wires. In addition, the leak rate of the feedthrough along a flat seal surface, under pressure or vacuum, is also significantly reduced. Furthermore, the product design allows quick assembly and easy identification of the sensor conductor wires prior to soldering or crimping to the pins of a PC connector.
Alternatively, thin film metal interconnect conductor lines and thin film RTD or thermocouple sensor elements for measuring process temperature can be directly deposited on a glass substrate or on a dielectric film deposited on a silicon wafer or other conductive substrate. Measuring temperature directly on the substrate surface can reduce many sources of measurement error. Also, simple surface protection of the thin film interconnect and sensor elements enables easier measurement of wet processes. The primary improvement of temperature measurement accuracy in thin film instrumented wafers is due to much higher thermal conduction between the substrate and sensor. The benefit is greatest when measuring substrate temperature in a non-isothermal heating environment.